Textured skin sheets for internal automotive furnishings, such as instrument panels, door trim and so on, were made principally of soft polyvinyl chloride resin (PVC). In recent years, however, olefinic thermoplastic elastomers have been used. Textured skins of olefinic thermoplastic elastomers may be produced by a number of techniques including positive and negative vacuum forming. Positive vacuum forming is carried out using a non-grained forming mold having perforations for evacuating the interface between the mold surface and the textured skin sheet (texture is formed on the outer face of an extruded sheet by embossing prior to vacuum forming) inside face by placing the texture or grain sheet on the mold with its textured outer face being exposed to the exterior and pressing the sheet onto the mold face by evacuating the space beneath the sheet. Therefore, the patterns of the textured outer face of the textured sheet may be apt to undergo deformation during the process of forming. In particular, a textured skin made of an olefinic thermoplastic elastomer may suffer from such deformation of the pattern upon a vacuum forming, especially upon positive vacuum forming, more easily as compared with that made of soft PVC, resulting in an inferior ability for retaining the texture.
Attempts to improve grain retention in olefin thermoplastic elastomers include, for example, peroxide treatment of polyolefin elastomer rich polypropylene blends subjected to dynamic heat treatment to increase blend viscosity and lower flexural modulus, thus promoting grain retention. For example, see U.S. Pat. Nos. 4,861,834; 5,218,046; 5,852,100; 6,100,335; 6,500,900; and 6,565,986. However, this approach involves reactive extrusion of the polyolefin elastomer blend and is difficult to control the degree of radical reactions. Thermoforming may also be impaired due to the elastomer cross-linking.
An alternative approach for polyolefin elastomer rich blends has involved electron beam cure. For example, see U.S. Pat. No. 7,534,318. This approach generally improves grain retention, but is limited to producers with access to expensive e-beam technology and is a rather expensive solution to the problem.
There is still a need for an improved cross-linkable polyolefin composition and simple process which provides improved grain retention when a sheet of said composition is thermoformed/vacuum formed into a textured skin.